1. Field of the Invention
The present invention relates generally to a digital receiver circuit. More particularly to a photo receiver circuit in a form of IC for an optical data link.
2. Description of the Related Art
In a transmitter and receiver of an optical data link, speeding-up, down-sizing, lowering of cost, power saving, providing wider reception dynamic range corresponding to a transmission line of plastic optical fiber being inexpensive and having large loss, have been demanded.
A construction of the conventional wide reception dynamic range photo receiver circuit is illustrated in FIG. 5. The shown conventional circuit is constructed with a photodiode 1, a pre-amplifier portion 15 and a post amplifier portion 16. Then, the pre-amplifier portion 15 and the post amplifier portion 16 are connected by way of an alternating current coupling employing a capacitor 23.
The pre-amplifier portion 15 has been disclosed in Japanese Unexamined Patent Publication (Kokai) No.Showa 62-257204. The pre-amplifier portion 15 is constructed with an inverting type transimpedance amplifier 11, an npn type transistor 3 having an emitter connected to an input end of the transimpedance amplifier 11 and a collector connected to a positive power source, an npn type transistor 4 having a collector connected to an input end of the transimpedance amplifier 11 and an emitter connected to a negative power source, a constant voltage source 19 and an operational amplifier 20 performing non-inverting amplification and level shifting for an output of the transimpedance amplifier 11, a peak detection circuit 17 detecting and holding a peak value of the output of the operational amplifier 20 and outputting to a base of the transistor 3, a constant voltage source 21 and an operational amplifier 22 performing inverting amplification and level shifting for an output of the transimpedance amplifier 11, and a peak detection circuit 18 detecting and holding a peak value of the output of the operational amplifier 22 and outputting to a base of the transistor 4.
The post amplifier portion 16 is constructed with a differential output limiting amplifier 24 and a quantizer 14 outputting one of a logical "1" and a logical "0" depending upon polarity of a differential voltage of a positive-phase output and a negative-phase output of the limiting amplifier 24.
Next, discussion will be given for an operation of the pre-amplifier portion 15 of the conventional photo receiver circuit with reference to FIG. 6. FIG. 6 shows a relationship between an amplitude of a photo current pulse flowing into the photodiode 1 and an output voltage at a point B. A bias is applied so that both transistors 3 and 4 are turned OFF (Tr3 off, Tr4 off) when an amplitude of the input current is small, for lowering of noise. Therefore, operation of the pre-amplifier portion 15 becomes linear operation of the inverting type transimpedance amplifier 11. According to increasing of the amplitude of the input current, a voltage BM at a point B corresponding to a mark portion drops and a voltage BS at a point B corresponding to a space portion becomes constant.
When the amplitude of the input current increases, in an offset control loop constructed with a transistor 4, a peak detector 18, a constant voltage source 21 and an operational amplifier 22, the voltage corresponding to the mark portion is detected by the peak detection circuit 18. By this, the transistor 4 turns ON (tr4 on) to maintain the voltage at the point B corresponding to the mark portion. Thus, according to increasing of the amplitude of the input current, the voltage of the point B corresponding to the space is elevated.
When the amplitude of the input current is further increased, in addition to the operation of the foregoing offset control loop, in an input impedance control loop constituted of a transistor 3, a peak detector 17 a constant voltage source 19 and an operational amplifier 20, a voltage corresponding to the space portion is detected by the peak detection circuit 17. By this, the transistor 3 is turned ON (Tr3 on) to lower an input impedance. Accordingly, an alternating current component of an input current pulse flows to the transistor 3 and a direct current component flows to the transistor 4 so that quite limited amount is input to the transimpedance amplifier 11. Thus, an automatic gain control is performed for maintaining the voltages BM and BS at the point B respectively corresponding to the mark portion and the space portion constant even if the amplitude of the input current is increased. By this, comparing with the transimpedance amplifier 11, a dynamic range of the input can be significantly increased.
In the above-mentioned conventional photo receiver circuit encounters the problems to cause difficulty in integration of the pre-amplifier portion and the pose amplifier portion into a single chip and in reduction of number of parts which are inherent for down-sizing, lowering of power consumption and for lowering of price.
Namely, in order to achieve integration into a single chip, it becomes necessary for establishing DC coupling for all of the circuits. However, in the conventional receiver circuit, since the DC level of the output of the pre-amplifier 15 is variable depending upon the amplitude of the input current, an alternating coupling using an external capacitor 23 externally applied to the IC becomes necessary between the pre-amplifier portion and the post amplifier. Therefore, number of parts and power consumption can be increased.